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Disease Jumped from Pigs to Humans with Rearing Changes

02 April 2015

UK - The move to rearing pigs in large, indoor groups occurred at the same time as the emergence of bacterial meningitis, a disease affecting both pigs and humans, new research from the University of Cambridge shows.

Streptococcus suis is the leading cause of bacterial meningitis in adult humans in parts of southeast Asia and in pigs around the world.

Almost every pig carries harmless strains of the bacterium. However, a more virulent group of strains of the bacteria also exist, which cause disease in pigs worldwide and are a major driver of antibiotic use for prevention.

Increasingly, this group of strains is also implicated in serious human diseases such as meningitis and septicaemia.

The international study found that the harmless strains and disease-causing strains differed genetically. In particular, the disease-causing strains have between 50 and 100 fewer genes than the commensal strains.

Dr Lucy Weinert from the Department of Veterinary Medicine at the University of Cambridge, first author of the study, said: “It seems that the loss of genes is associated with causing disease.

"This is something we see quite often in bacteria, but for reasons that are unclear.”

By examining how the S. suis bacteria have evolved over time, the researchers were able to show that the emergence of a group of strains causing meningitis in pigs and the human form of the disease dates back to the 1920s.

The coincides with the introduction of wide-scale indoor rearing of meat-producing pigs in larger groups, supported by government schemes that favoured larger producers with regular throughput.

“A group of more virulent strains seem to have emerged at around the time the pig industry changed, and it is these strains that mostly cause disease in pigs and humans,” said Professor Duncan Maskell, Head of the School of the Biological Sciences at Cambridge.

“Human S. suis disease in the West is extremely rare, but is seen more frequently in south east Asia.

"It is most likely spread to humans through poor food hygiene practices or other environmental factors. This emphasises the importance of monitoring such practices and putting policies in place to reduce the risk of the spread of infections between species.”